Infra-red switchable mixture for producing lithographic printing plate

ABSTRACT

Water-based mixtures that can be applied to substrates to produce infra-red sensitive layers with hydrophilic surfaces, which may be switched by image-wise exposure in an infrared imaging plate-setter to give oleophilic printing areas, for producing processless offset lithographic printing plates.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This patent application claims priority from and is related to U.S.Provisional Patent Application Ser. No. 60/444,182, filed Feb. 3, 2003,this U.S. Provisional Patent Application incorporated by reference inits entirety herein.

FIELD OF INVENTION

This invention relates to lithographic printing plates which are imagedby switching the exposed areas of the plate surface from hydrophilic tooleophilic properties and which do not need subsequent processing beforeuse as conventional (wet) offset lithographic printing plates.

BACKGROUND OF THE INVENTION

Offset lithographic printing has remained a most popular method ofprinting for many years. An important reason for this is the relativeease with which offset lithographic printing plates can be produced. Formany years, the most widely used method for plate preparation has beenthat which utilizes specially prepared masking films through whichpre-sensitized printing blanks are selectively hardened or softened(according to the chemistry of the plate) by exposure to ultra violetlight. The plate then undergoes a development process, during which themore soluble regions of the plate are washed away using a processingliquid, leaving the remainder of the coating to function as the printareas.

In recent years, various considerations have arisen that point toadvantages for modification of hitherto generally accepted practices.With the advent of computers, information for printing is prepareddigitally and it has become preferable to use this digital informationas directly as possible in plate preparation. One obvious way is toeliminate the masking film. Not only are these films a source ofexpense, but the most widely used films are based on silver chemistry,whereby the exposure and handling of the film must be in a lightexcluding environment and where chemical solutions which are unstable,messy and environmentally problematic are used. One answer is to befound in computer-to-plate (CTP) systems whereby plates are imaged witha light source which is modulated directly to correspond to the digitalinformation from the computer. A commercial type of CTP that is nowgenerally available is that based on thermal imaging due to theavailability of relatively inexpensive laser sources for infra-redradiation. Such machines have become increasingly popular and now have asignificant share of the plate-making market.

A further trend in the printing market should be noted. The number ofcopies required for any particular printing job is declining. There arevarious reasons for this. They include the high cost of warehousing, andthe trend to more local printing to avoid high postage. This has beentermed “distribute and print” as opposed to the former practice of“print and distribute.” Thus there is a growing market for short runprinting.

Laser imaging of offset printing blanks is well known. U.S. Pat. No.4,054,094 to Caddell et al describes such a system. The plate comprisesa thin hard hydrophilic coating on a polymer coated on a polymeric ormetal base. The laser etches (or ablates) away the layers to expose thebase material which is oleophilic. The laser used is a high energycarbon dioxide one—available at the time of filing of the Cadell patent.The patent would not be applicable for use with the modern lower energy,lower costs laser diodes.

U.S. Pat. No. 4,693,958 to Schwartz et al describes a plate whereby thewater-soluble coating is hardened by the laser radiation and theunhardened non image areas are washed away to expose a hydrophilicaluminum substrate. The film thickness to be hardened is at least 0.2microns and here again the energy needed is high.

U.S. Pat. No. 3,511,178 to Curtin introduced a new concept of“driographic” or waterless printing. This uses offset printing plateswith ink-repelling surfaces which do not require fount. As this is aconcept that will be further referred to within the present application,it is useful and convenient to distinguish between hitherto conventionaloffset lithographic printing plates and the relatively newer waterlessoffset lithographic printing plates. The conventional plates will bereferred to as “wet” because such plates need a fount solution to beapplied to their surface before ink application, during the printingprocess. Fount solutions are water-based. Hence the term “wet.

U.S. Pat. No. 5,339,737 to Lewis et al describes both wet litho andwaterless litho, wherein the upper layer or layers of the plate areablated away. The upper layer is either oleophobic for waterless platesor hydrophilic for conventional wet process plates. The substrate isoleophilic in both cases.

U.S. Pat. No. 5,353,705 to Lewis et al is based on a technology similarto the previous patent but describes additional layers for secondarypartial ablation.

U.S. Pat. No. 5,487,338 to Lewis et al is another patent of the samefamily but includes reflective layers. Multiple layered plates areexpensive to produce and it is more difficult to maintain a consistentstandard of quality in manufacture.

U.S. Pat. No. 5,493,971 to Lewis et al describes wet plates with threelayers. The top layer is ablated during imaging and the secondprotective layer is then washed away to reveal hydrophilic aluminum.

In general, where ablation is used for imaging, it is necessary toremove the ablation products. In some cases they may be collected byvacuum, but in general some detritus remains on the plate and must beremoved by dry or wet rubbing.

In contrast to ablation, U.S. Pat. No. 5,401,611 to Edwards Sr. et aldescribes a plate with a very thin silver-based oleophobic layer on topof a hydrophilic layer. Infra red laser radiation converts theoleophilic surface to being hydrophilic. The chemistry involved here isbased on silver and the plate manufacturing preparation is complex. Thispatent represents an early attempt to produce a plate that isprocessless and where the surface of the plate is switched by imagingradiation from oleophilic to hydrophilic properties. A polymeric surfacewhere the change of water or ink receptive properties under theinfluence of imaging radiation is now referred to as a switchablepolymer and a plethora of patent applications makes manifest theinterest in this.

Processless switchable polymer plates or surfaces have severaladvantages. The elimination of processing shortens the plate-making timeand is more economic in saving on purchase of processing equipment andthe use and disposal of processing liquids. Such liquids are usuallywater-based with a high pH and are environmentally problematic in theirdisposal. In addition, processing chemistry usually shows deteriorationover a period of days or weeks as it becomes contaminated with thewashed out material from the plates, or loses its efficacy by reactionwith carbon dioxide in the air. The processor often requires periodiccleaning as plate sludge accumulates.

Offset printing machines have now been developed that have imagingon-press, where ideally the plate material has no processing or wheresuch processing is very simple. An example of this technology is theHeidelberg Model QMDI-46, manufactured by Heidelberg Druckmaster AG(Germany), which utilizes infrared ablatable waterless plates.Furthermore, Gelbart (U.S. Pat. No. 5,713,287) has proposed the use ofthe cylinder itself as the plate substitute substrate, whereby aswitchable polymer is then sprayed onto the surface, imaged and thenused as a printing form. After printing, the coating is removed and theprocess may be repeated for the next print run. It is preferable thatthe sprayed polymer contains all of the imaging and printing propertiesin a single coating.

Yu in WO 02/051636 A1 describes an invention and notes the history ofits development whereby an infrared sensitive water-based emulsion isapplied to a hydrophilic substrate, dried by evaporation of the waterand imaged. Imaging is by emulsion coalescence, which produces insolubleoleophilic regions. Processing is done using the fount to remove theuncoalesced, unimaged areas on press. Such a method contaminates thefount solution, which in general in offset lithographic printing iscarefully controlled to give optimum print quality and machinerunability.

Zheng et al (U.S. Pat. No. 6,413,694) describes switchable polymerlayers whereby oleophilic layers are switched to hydrophilic surfaces,thus describing a positive working switchable polymer plate. InUS2002/0142245 A1, Zheng describes the opposite type of switching fromhydrophilic to hydrophobic. Examples given in this patent show theutilization of special syntheses to produce key materials for theinvention.

Morgan (U.S. Pat. No. 6,063,528) describes switchable polymer layerswhereby hydrophilic layers containing a metallic salt of a fatty acidsuch as silver behenate and a water-soluble polymer, as well as infraredabsorbable dyes, are switched by imaging to oleophilic surface areas.The use of silver in the plate makes it relatively expensive.

SUMMARY OF THE INVENTION

It is an objective of the invention to produce mixtures that can beapplied to substrates to produce infrared sensitive layers withhydrophilic surfaces, which may be switched by image-wise exposure in aninfrared imaging plate-setter to give oleophilic printing areas.

Thus, there is provided a mixture for producing infra-red sensitivelayers with hydrophilic surfaces, which may be switched by image-wiseexposure to give oleophilic printing areas, comprising: polyacrylicacid; polyvinyl alcohol; aminoplast cross-linking agent; infraredsensitive dye or pigment; and acidic hydrophobic polymeric emulsion,wherein said mixture is deposited from an aqueous carrier liquid.

The mixture may additionally comprise at least one of wetting agents,fillers and exotherms.

In one embodiment of the present invention, the polyacrylic acid and thepolyvinyl alcohol comprise together between 20% and 58% of the totalsolids of the mixture.

In another embodiment of the present invention, the polyvinyl alcoholcomprises less than 15% of the total solids of the polyacrylic acid.

In another embodiment of the present invention, the aminoplast resin iswater-soluble.

In another embodiment of the present invention, the aminoplast resinprovides crosslinking of the polyacrylic and polyvinyl alcohol resins tohelp insolublize them.

In an additional embodiment of the present invention, the mixture doesnot contain catalysts for the cross-linking process involving theaminoplast.

In another embodiment of the present invention, the aminoplast does notexceed 40% of the polyvinyl alcohovpolyacrylic acid mixture.

The aminoplast may be methyl methylol urea.

The dye may be water-soluble.

In yet another embodiment of the present invention, the infraredabsorbing pigment comprises water-dispersed carbon black and the dyesdissolve in water miscible solvents.

It is a further objective of the invention to produce an offsetlithographic printing blank using raw materials that are readilyavailable and can be purchased off the shelf and do not require costlysynthesis.

It is a further objective of the invention to produce an offsetlithographic printing plate which after imaging requires no furthertreatment before printing.

It is a further objective of the invention to produce an offsetlithographic printing plate that after imaging and when placed on aprinting press does not require the fount to wash away the backgroundcoating of the plate.

It is a further objective of the invention to produce mixtures which canbe applied as a single coat to produce switchable polymer offsetlithographic plates that can be imaged in an infrared laserplate-setter.

It is a further objective of the invention to provide aqueous mixturesof switchable polymer material that can be applied to the surface of acylinder of a printing press, the water evaporated off on-press, imaged,printed and then the coating completely removed and a further cycle doneto constitute a plateless system.

Thus, there is provided a method of producing a processless offsetlithographic printing plate, comprising the steps of: providing amixture comprising polyacrylic acid; polyvinyl alcohol; aminoplastcross-linking agent; infrared sensitive dye or pigment; and acidichydrophobic polymeric emulsion, said mixture deposited from an aqueouscarrier; applying a layer of the mixture onto a substrate to produce aninfra-red sensitive layer with a hydrophilic surface; drying the appliedlayer; and switching the hydrophilic surface by image-wise exposure toinfrared imaging to give oleophilic printing areas.

The substrate may comprise an offset lithographic printing plateprecursor or the surface of a printing press cylinder.

The mixture may additionally comprise at least one of wetting agents,fillers and exotherms.

According to one embodiment of the present invention, the polyacrylicacid and the polyvinyl alcohol comprise together between 20% and 58% ofthe total solids of the mixture.

According to another embodiment of the present invention, the polyvinylalcohol comprises less than 15% of the total solids of the polyacrylicacid.

According to another embodiment of the present invention, the aminoplastresin is water-soluble.

According to yet another embodiment of the present invention, theaminoplast resin provides crosslinking of the polyacrylic and polyvinylalcohol resins to help insolublize them.

According to yet another embodiment of the present invention, themixture does not contain catalysts for the cross-linking processinvolving the aminoplast.

According to yet another embodiment of the present invention, theaminoplast does not exceed 40% of the polyvinyl alcohol/polyacrylic acidmixture.

The aminoplast may be methyl methylol urea.

The dye may be water-soluble.

The infrared absorbing pigment may comprise water-dispersed carbonblack.

DETAILED DESCRIPTION OF THE INVENTION

The mixtures used in this invention are all aqueous based. The aqueoussolution used in this invention contains a number of essentialcomponents. These are as follows:

1. Polyacrylic acid

2. Polyvinyl alcohol

3. Aminoplast cross-linking agent.

4. Infrared sensitive dye or pigment.

5. Acidic polymeric emulsion.

6. There are also optional additional ingredients such as wettingagents, fillers and exotherms.

It is important also to emphasize that the mixtures must not containcatalysts for the cross-linking process involving the aminoplast.

The type of substrate coating used is described in co-pending U.S.Provisional Application No. 60/444,183 entitled CTP-Inkjet UsingSwitchable Polymer, commonly owned with the present application andhereby incorporated by reference. The polyacrylic acid and the polyvinylalcohol comprise the water-soluble elements of the formulation thatprovide the hydrophilic nature of the coating. These together mustconstitute between 20% and 58% of the total solids of the mixture asapplied to the substrate. The polyvinyl alcohol must be less than 15%(weight of solids) of the polyacrylic acid. The aminoplast resin must bea water-soluble one and when the coating is dried probably provides somecrosslinking of the polyacrylic and polyvinyl alcohol resins to helpinsolublize them. However, whilst aminoplasts are not generally usedwithout a catalyst which may be, for instance a sulfonic acid, in thissystem no catalyst must be present. In trials using such catalysts itwas found that drying temperature was critical in achieving the balanceof water resistance of the coating against hydrophobic properties. Insuch formulations, it was found that it is not possible to achievecomplete water resistance and hydrophilic properties merely by thebalance between the polyacrylic/polyvinyl alcohol mixture and theaminoplast. Too much aminoplast or too high a drying temperature causessufficient cross-linking to turn the entire coating oleophilic. Toolittle aminoplast and too low a drying temperature causes the coating tobe water soluble and consequently the fount will remove the entirecoating during the printing process. Without the presence of thecatalyst and in the formulations as described in this application, itwas possible to achieve excellent hydrophilic properties together withexcellent water resistance and adhesion to substrate, where the finishedformulation is subject to sufficient heating.

The aminoplast must not exceed 40% of the polyvinyl alcohol/polyacrylicacid mixture, otherwise the dried coating will lose its hydrophilicity.An examples of a suitable aminoplast is methyl methylol urea. Also, ithas been found that it is essential to incorporate an acidicwater-resistant water-based emulsion into the formulation. It issomewhat unexpected that such a material can be used without ruining thesurface hydrophilic properties of the coating whilst givingfount-resisting properties. It is not clear whether the excellentadhesion of the layer to the substrate and resistance to fount attack isa result of the presence of the hydrophilic emulsion or of someuncatalysed crosslinking that may take place between the aminoplast andthe —OH groups from the polyvinyl alcohol and the polyacrylic acid.

Switching of the coating from being hydrophilic to oleophilic occurs inthe image areas when they are subject to the heat produced by laserimaging. It is most likely that this occurs by heat initiatedcross-linking between the aminoplast and other ingredients.

Suitable infrared dyes and pigments are those known to the art. Thesemay be used individually or in mixtures. The preferred type of dye isthat which is water-soluble and examples of such dyes are water-solublenigrosine, ADS830W (sold by American Dye Source Incorporated, Quebec,Canada) and NK 5042 (sold by Hayashibara Biochemical laboratories,Kakoh-Shikiso Institute, Okayama, Japan). However, it is also possibleto use water dispersed carbon black and dyes dissolved in water misciblesolvents, such as alcohol, and then added to form a solution ordispersion into the aqueous mixture.

The following example is given to illustrate the invention.

EXAMPLE I

The following formulation was made up—all quantities are in parts byweight. All of the ingredients shown below are readily available and canbe purchased off the shelf. Polyvinyl Alcohol Solution 7.2 grams (12% inwater) Polyacrylic Acid 18.4 grams (35% solution in water) Deionisedwater 71 grams BYK 346 1.9 grams Walpol 40-136 25.15 grams Cymel UFR-602.04 grams Nigrosine 1.8 gramsThe mixture was high-speed mixed and coated with a Mayer rod onto 150micron thick degreased aluminum foil. It was dried in an oven at 160° C.for 4 minutes and had a coating weight of approximately 7 grams persquare meter. The finished plate was then imaged at approximately 900milli-joules per square centimeter on a Creo Lotem Flexo plate-setter.The imaged plate was taken directly from the plate-setter and mounteddirectly onto a Heidelberg GTO offset lithographic printing press and8000 good printing impressions run off using the conventional wet offsetprocess.

In addition, the above formulation can be sprayed onto an aluminumsurface of a plate cylinder in a printing press and the coating driedand fused onto the surface at 160° C. and then imaged and used forprinting as described by Gelbart in U.S. Pat. No. 5,713,287, which isco-owned and incorporated herein by reference.

Sources of Raw Materials

BYK 346. BYK-Chemie GmbH, Posffach 100245, Wesel

Cymel UFR-60 Cytec Industries. Five Garret Mountain Plaza, WestPatterson, N.J. USA

Walpol 40-136. Reichold, Inc, Research Triangle Park, N.C., USA.

1. A mixture for producing infra-red sensitive layers with hydrophilicsurfaces, which may be switched by image-wise exposure to giveoleophilic printing areas, comprising: polyacrylic acid; polyvinylalcohol; aminoplast cross-linking agent; infrared sensitive dye orpigment; and acidic hydrophobic polymeric emulsion, wherein said mixtureis deposited from an aqueous carrier liquid.
 2. The mixture of claim 1,additionally comprising at least one of wetting agents, fillers andexotherms.
 3. The mixture of claim 1, wherein the polyacrylic acid andthe polyvinyl alcohol comprise together between 20% and 58% of the totalsolids of the mixture.
 4. The mixture of claim 1, wherein the polyvinylalcohol comprises less than 15% of the total solids of the polyacrylicacid.
 5. The mixture of claim 1, wherein the aminoplast resin iswater-soluble.
 6. The mixture of claim 1, wherein the aminoplast resinprovides crosslinking of the polyacrylic and polyvinyl alcohol resins tohelp insolublize them.
 7. The mixture of claim 1, wherein the mixturedoes not contain catalysts for the cross-linking process involving theaminoplast.
 8. The mixture of claim 1, wherein the aminoplast does notexceed 40% of the polyvinyl alcohol/polyacrylic acid mixture.
 9. Themixture of claim 1, wherein the aminoplast is methyl methylol urea. 10.The mixture of claim 1, wherein the dye is water-soluble.
 11. Themixture of claim 1, wherein the infrared absorbing pigment compriseswater-dispersed carbon black and wherein the dyes dissolve in watermiscible solvents.
 12. A method of producing a processless offsetlithographic printing plate, comprising the steps of providing a mixturecomprising polyacrylic acid; polyvinyl alcohol; aminoplast cross-linkingagent; infrared sensitive dye or pigment; and acidic hydrophobicpolymeric emulsion, said mixture deposited from an aqueous carrier;applying a layer of the mixture onto a substrate to produce an infra-redsensitive layer with a hydrophilic surface; drying the applied layer;and switching the hydrophilic surface by image-wise exposure to infraredimaging to give oleophilic printing areas.
 13. The method of claim 12,wherein said substrate comprises an offset lithographic printing plateprecursor.
 14. The method of claim 12, wherein said substrate comprisesthe surface of a printing press cylinder.
 15. The method of claim 12,wherein said mixture additionally comprises at least one of wettingagents, fillers and exotherms.
 16. The method of claim 12, wherein thepolyacrylic acid and the polyvinyl alcohol comprise together between 20%and 58% of the total solids of the mixture.
 17. The method of claim 12,wherein the polyvinyl alcohol comprises less than 15% of the totalsolids of the polyacrylic acid.
 18. The method of claim 12, wherein theaminoplast resin is water-soluble.
 19. The method of claim 12, whereinthe aminoplast resin provides crosslinking of the polyacrylic andpolyvinyl alcohol resins to help insolublize them.
 20. The method ofclaim 12, wherein the mixture does not contain catalysts for thecross-linking process involving the aminoplast.
 21. The method of claim12, wherein the aminoplast does not exceed 40% of the polyvinylalcohol/polyacrylic acid mixture.
 22. The method of claim 12, whereinthe aminoplast is methyl methylol urea.
 23. The method of claim 12,wherein the dye is water-soluble.
 24. The method of claim 12, whereinthe infrared absorbing pigment comprises water dispersed carbon black.